Petroleum refining and petrochemical processes frequently involve the processing of fluids over particulate solids contained within a pressure vessel. Internal partitions can subdivide the interior of a pressure vessel into different chambers to permit staged or multiple contacting operations within a single vessel. These partitions routinely take the form of, or are used in conjunction with, collection or distribution grids. Process requirements, such as the collection and distribution of fluids, regularly dictate the employment of relatively flat partitions. Concomitantly, pressure vessels usually are closed by rounded “heads” at each end. The rounded head and flat interior partitions at each end of a vessel create a head space whose configuration is not suited to process purposes, risking contamination or deterioration of the process if this remains a dead volume.
Further, flat partitions are subject to structural damage from differential pressures of as little as 15 kPa or even less across the partition. Structural damage to a partition has the potential to create leaks across the partition or in associated distribution/collection piping.
Thus, maintaining structural integrity of interior end partitions requires pressure balancing between the head space and the adjoining volume on the process side of the partition. The head space can serve as an equalization chamber through a small opening or port in the partition communicating head fluid to and from the process chamber on the opposite side of the partition. However, this arrangement risks some inefficiency in the process through head fluid passing into the process chamber and some loss in yield through process fluid passing into the head space.
A specific technology which illustrates the above problem is the simulated moving bed (“SMB”) adsorbent process described in U.S. Pat. No. 2,985,589. The process distributes and collects process streams from multiple chambers of adsorbent defined by internal partitions located within a pressure vessel and arranged as distribution/collection grids. Periodic shifting of the input and effluent streams through the chambers simulates movement of the adsorbent and permits delivery or withdrawal of the streams with a desired concentration profile. Delivering or withdrawing these process streams requires flat distribution grids.
The head space resulting from the flat distribution grids and a concave end is flushed by a small flow of a flush fluid, usually comprising a desorbent material, which equalizes pressure across the distribution grids. A desorbent material normally is selected so that passage of this material into the adsorbent bed through a grid opening does not contaminate the products of the process. However, the periodic shifting of the input and effluent streams through the chambers of adsorbent can effect a buildup of contaminants in the desorbent through leakage through the grid, particularly in the bottom head of the chamber. Further, the addition of desorbent to the adsorbent bed through the grid opening can interfere with the optimization of purity and recovery by taking up adsorbent capacity and hindering an accurate accounting of flow through the adsorbent beds.
U.S. Pat. No. 5,595,665, incorporated herein in its entirety by reference thereto, addresses these issues by channeling the fluid generated by a head flush into a low volume chamber (referred to herein as “snorkel”) in the head space and withdrawing fluid from the pressure vessel through the snorkel. Withdrawing fluid generated by the head flush and channeling the fluid through the snorkel reduces or eliminates the circulation of fluid between the equalization chamber and the adjacent process chamber and minimizes the amount of contamination that can result from any circulation of fluid resulting from pressure fluctuations. The withdrawal of fluid through the snorkel also provides a non-contaminating path for withdrawing leakage from the equalization chamber of the vessel.
However, this feature fails to address how to prevent the loss of valuable product in the fluid withdrawn from the snorkel.